[JITLink][MachO] Handle MachO section start/end symbols.

Transform section$start$<section-name> and section$end$<section-name> external
symbols into defined symbols when a section named <section-name> is present.

rdar://125357048

Coding my way home: 8.98112N, 79.52094W

Author: lhames

llvm/lib/ExecutionEngine/JITLink/DefineExternalSectionStartAndEndSymbols.h

@@ -127,6 +127,29 @@ identifyELFSectionStartAndEndSymbols(LinkGraph &G, Symbol &Sym) {
   return {};
 }
 
+/// MachO section start/end symbol detection.
+inline SectionRangeSymbolDesc
+identifyMachOSectionStartAndEndSymbols(LinkGraph &G, Symbol &Sym) {
+  constexpr StringRef StartSymbolPrefix = "section$start$";
+  constexpr StringRef EndSymbolPrefix = "section$end$";
+
+  auto SymName = Sym.getName();
+  if (SymName.starts_with(StartSymbolPrefix)) {
+    auto [SegName, SecName] =
+      SymName.drop_front(StartSymbolPrefix.size()).split('$');
+    std::string SectionName = (SegName + "," + SecName).str();
+    if (auto *Sec = G.findSectionByName(SectionName))
+      return {*Sec, true};
+  } else if (SymName.starts_with(EndSymbolPrefix)) {
+    auto [SegName, SecName] =
+      SymName.drop_front(EndSymbolPrefix.size()).split('$');
+    std::string SectionName = (SegName + "," + SecName).str();
+    if (auto *Sec = G.findSectionByName(SectionName))
+      return {*Sec, false};
+  }
+  return {};
+}
+
 } // end namespace jitlink
 } // end namespace llvm
 

llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp

@@ -14,6 +14,7 @@
 #include "llvm/ExecutionEngine/JITLink/DWARFRecordSectionSplitter.h"
 #include "llvm/ExecutionEngine/JITLink/aarch64.h"
 
+#include "DefineExternalSectionStartAndEndSymbols.h"
 #include "MachOLinkGraphBuilder.h"
 
 #define DEBUG_TYPE "jitlink"
@@ -593,6 +594,11 @@ void link_MachO_arm64(std::unique_ptr<LinkGraph> G,
     Config.PrePrunePasses.push_back(createEHFrameSplitterPass_MachO_arm64());
     Config.PrePrunePasses.push_back(createEHFrameEdgeFixerPass_MachO_arm64());
 
+    // Resolve any external section start / end symbols.
+    Config.PostAllocationPasses.push_back(
+        createDefineExternalSectionStartAndEndSymbolsPass(
+            identifyMachOSectionStartAndEndSymbols));
+
     // Add an in-place GOT/Stubs pass.
     Config.PostPrunePasses.push_back(buildTables_MachO_arm64);
   }

llvm/lib/ExecutionEngine/JITLink/MachO_x86_64.cpp

@@ -14,6 +14,7 @@
 #include "llvm/ExecutionEngine/JITLink/DWARFRecordSectionSplitter.h"
 #include "llvm/ExecutionEngine/JITLink/x86_64.h"
 
+#include "DefineExternalSectionStartAndEndSymbols.h"
 #include "MachOLinkGraphBuilder.h"
 
 #define DEBUG_TYPE "jitlink"
@@ -516,6 +517,11 @@ void link_MachO_x86_64(std::unique_ptr<LinkGraph> G,
     else
       Config.PrePrunePasses.push_back(markAllSymbolsLive);
 
+    // Resolve any external section start / end symbols.
+    Config.PostAllocationPasses.push_back(
+        createDefineExternalSectionStartAndEndSymbolsPass(
+            identifyMachOSectionStartAndEndSymbols));
+
     // Add an in-place GOT/Stubs pass.
     Config.PostPrunePasses.push_back(buildGOTAndStubs_MachO_x86_64);
 

llvm/test/ExecutionEngine/JITLink/AArch64/MachO_section_start_and_stop_symbols.s

@@ -0,0 +1,30 @@
+# RUN: llvm-mc -triple=arm64-apple-darwin24 -filetype=obj -o %t.o %s
+# RUN: llvm-jitlink -noexec -check %s %t.o
+
+# jitlink-check: *{8}_z = (*{8}_y) + 4
+
+	.section	__TEXT,__text,regular,pure_instructions
+	.globl	_main
+	.p2align	2
+_main:
+        mov     w0, #0
+        ret
+
+        .section        __DATA,__custom_section
+	.globl	_x
+	.p2align	2, 0x0
+_x:
+	.long	42
+
+	.section	__DATA,__data
+	.globl	_y
+	.p2align	3, 0x0
+_y:
+	.quad	section$start$__DATA$__custom_section
+
+	.globl	_z
+	.p2align	3, 0x0
+_z:
+	.quad	section$end$__DATA$__custom_section
+
+.subsections_via_symbols